1. Field of the Invention
The present invention relates to a high-frequency amplifier in which discrete transistor input/output terminals, with which radiation fins having a function of an electric ground terminal come into tight contact, and matching microstrip lines on the input/output dielectric boards are connected to each other and, more particularly, to circuit mounting which aims at removing unstable factors in the amplifier characteristics caused by the parasitic inductance produced on the high-frequency ground line due to high-frequency discontinuity of the ground surface near the transistor input/output terminals.
2. Description of the Prior Art
As an example of the high-frequency amplifier of this type, for example, a high-frequency power amplifier described in Japanese Unexamined Utility Model Publication No. 62-21618 is known. A conventional high-frequency amplifier (which will be referred to as the first conventional technique) will be described with reference to FIG. 1A. In the example shown in FIG. 1A, 50-.OMEGA. microstrip lines 9 and 21, and an input matching stab 46 and an output matching stab 20, are formed on a dielectric board 37. The microstrip lines 9 and 21, and the input matching stab 46 and output matching stab 20, are used as matching circuits. Holes are formed in the dielectric board 37, having two surfaces covered with copper thin films, around the mounting portion of a transistor 1, and the dielectric board 37 is attached on a common ground surface member 27 with attaching screws 28. The transistor 1 having radiation fins 4 is attached to the common ground surface member 27 with the attaching screws 28 through the holes described above. A transistor input terminal 2 and the input matching stab 46, and a transistor output terminal 3 and the output matching stab 20, are connected to each other by soldering.
As shown in FIG. 1A, the transistor 1 is formed with a plurality of via holes 39, and has ground patterns 11 and transistor emitter ground terminals 45. Voltage-cutting chip capacitors 15 and 25 are respectively arranged on the microstrip lines 9 and 21.
As another conventional high-frequency amplifier, one shown in FIG. 1B is also known (this will be referred to as the second conventional technique). In the example shown in FIG. 1B, the constituent elements identical to those in the example shown in FIG. 1A will be denoted by the same reference numerals as in FIG. 1A. In the second conventional technique, the high-frequency amplifier has first and second dielectric boards 6 and 17 each of which has two surfaces covered with copper thin films. A microstrip line 9 and an input matching stab 46 are formed on the first dielectric board 6, and a microstrip line 21 and an output matching stab 20 are formed on the second dielectric board 17. The first and second dielectric boards 6 and 17 are separately mounted on a common ground surface member 27 with a transistor 1 interposed between them. A transistor input terminal 2 and a transistor output terminal 3 are connected to the input matching stab 46 and the output matching stab 20, respectively, by soldering.
In the first and second conventional techniques described above, major matching circuits each using a distributed or concentrated constant must be arranged on the board immediately near the transistor input and output terminals 2 and 3 in order to achieve various types of matching states with the transistor 1. Usually, to bring the ground conductors on the lower surfaces of the dielectric boards 6 and 17 or the ground conductor on the dielectric board 37 into tight contact with the common ground surface member 27, screw fastening is employed, as described above. Hence, the screws 28 cannot but be arranged to avoid the input matching stab 46 and output matching stab 20 immediately near the transistor input and output terminals 2 and 3.
The positions of the screws are inevitably separated from the transistor input and output terminals 2 and 3. Accordingly, as shown in FIG. 2, contact aiming at achieving electrical ground between lower-surface ground conductors 19a and 19b located on the board end faces immediately under the transistor input and output terminals 2 and 3, respectively, and the common ground surface member 27 becomes poor (this contact is indicated as contact points 35a and 35b in FIG. 2).
In addition, also because of surface treatment (solder coating, solder plating, and the like) of the lower surface of the board and the surface microstructure of the common ground surface, not only discontinuity and uncertainty occur in the electrical ground state at the board discontinuous portion, but also unstable contact is caused by a temperature change.
When such unstable contact occurs, as shown in FIG. 2, a high-frequency ground return current 29 flows along a route longer than a forward current 30 because of the discontinuity of the ground line near the lower surfaces 19a and 19b of the transistor input/output connecting portions, and a reverse ground return current 29' occurs at the ground point 35a. When this is expressed by an equivalent two-terminal twin circuit, as shown in FIG. 3, parasitic inductances 36 are produced on the ground return lines immediately near the transistor input and output terminals 2 and 3 that are significant portions in the matching conditions. The inductance itself of each of the parasitic inductances 36 behaves unstably due to a temperature change and unstable contact (uncertainty of the contact points 35a and 35b in FIG. 2) during assembly, thereby changing the matching state. A change in matching state then brings about variations in frequency and temperature characteristics of gain, power, and noise, non-reproducibility of various characteristics, and unwanted oscillation in the amplifier. Referring to FIGS. 2 and 3, reference numeral 31 denotes a signal source; 32 and 33, 50-.OMEGA. input and output loads, respectively; and 34, ground.
As in the second conventional technique, emitter ground terminals 45 at the two ends of the signal input and terminals 2 and 3 of the discrete transistor 1 are sometimes directly connected to the lower-surface conductor of the second dielectric board 17 by soldering. For this purpose, the transistor 1 and the dielectric board 17 must be temporarily assembled. This temporary assembly is not conveniently performed. Since the common ground surface member 27 avoids the soldering portions of the transistor emitter ground terminals 45 on the lower surface of the dielectric board 17, a complicated structure results.
Depending on the shape of the transistor package, the ground return route cannot be minimized as a secondary effect of this method. Hence, the versatility lacks.
As described above, in the conventional high-frequency amplifier, when obtaining matching of a discrete transistor, although the major matching circuits must be arranged immediately near the transistor input/output connecting portions, the positions of screws necessary for mounting cannot but be inevitably set remote from the input/output connecting portions. As a result, ground of the lower surfaces of the input/output matching circuit portions that highly need ground becomes poor (weak), making the matching state unstable.
More specifically, in the conventional high-frequency amplifier, when bringing the lower-surface ground conductor of a microstrip board (dielectric board), where the discrete transistor input/output matching circuits are arranged, into tight contact with the common ground surface, screw fastening is employed. Electrical ground between the common ground surface and the board lower-surface ground conductors immediately under the input/output connecting portions that are significant for matching becomes discontinuous and unstable, and the various characteristics of the amplifier become unstable.
In addition, in the conventional high-frequency amplifier, since a transistor having a package with a special shape and terminals arranged in a special manner is used, continuity of ground at the matching portion is attained by directly soldering the emitter ground terminal to the ground conductor on the lower surface of the matching board. However, such an amplifier cannot be assembled easily and has a complicated mechanism, as described above.